Known as an example of techniques in this kind of field is an illumination device described in Patent Literature 1. In this conventional illumination device, flexible printed circuit boards constituting a plurality of segments are attached to the inside of a holding frame, so as to form a three-dimensional dome. Angles of surfaces on which light sources are placed in the respective boards are adjusted so that a specimen is irradiated with light in a desirable illuminance distribution.
Patent Literature 2 discloses an optical diagnostic system in which a plurality of excitation light emitting diodes are arranged in front of a camera such that their angles of illumination center axes with respect to the image pickup light axis differ from each other. Patent Literature 3 discloses an observation device in which a plurality of light sources are disposed within a hemispherical hood which is provided with an observation window on its upper part. This device forms the hood from an elastic material such as polyurethane. Patent Literature 4 discloses an illumination device in which LED arrays are arranged on a hemispherical substrate.